Approximately 1.3 million fractures occurred last year in the United States as a result of osteoporosis. A number of factors have been shown to affect bone density including estrogen, dietary calcium intake and absorption, and physical activity. Exercise has been associated with increases in spine and radius density as well as increases in total body calcium. This protocol is the first phase of a two-year study of the effects of exercise on bone density and body composition in subjects on high and low intakes of calcium. In the current study, the effects of weight-bearing exercise on utilization of dietary calcium will be examined. Aims of the study include: 1) assessment of the effects of weight-bearing exercise on net intestinal absorption and urinary and sweat losses of calcium and 2) determining whether any changes in calcium balance observed are associated with changes in levels of parathyroid hormone (PTH), 25-hydroxyvitamin D (25-OHvit D), 1,25 dihydroxy vitamin D, (1,25(OH)2D), growth hormone (HGH), calcitonin or somatomedin C, or with different basal levels of estrone and estradiol. Twenty healthy, sedentary, postmenopausal women will participate in a supervised, 10-week weight-bearing exercise program at an intensity of 70% VO2 max while on a constant self-selected intake of calcium, phosphate, magnesium, fiber, protein, and vitamin D. Calories will be adjusted to maintain a constant weight and 400 IU vitamin D will be taken daily to reduce variation in vitamin D intake. Before and after the fitness program the following parameters will be assessed: muscle strength testing by isometric force generation and by Cybex testing, functional capacity (VO2, max), body composition (K40), the above hormonal parameters, and calcium absorption. The latter will be estimated, after ingestion of Ca47 and a nonabsorbable stool marker Cr51, as the sum of whole body retention plus urinary and sweat losses of Ca47. In addition, post-exercise HGH levels as well as 1,25(OH)2 D levels will be measured every two weeks throughout. Baseline bone density will be measured by dual photon absorptiometry. Using these techniques, potential effects of weight-bearing exercise on utilization of dietary calcium can be determined.